Abstract
The synchronization of immune responses is crucial for overall health, with disruption implicated in various human diseases. At the core of the antiviral defense system lies the receptor protein Retinoic acid-Inducible Gene I (RIG-I), essential for the clearance of viruses from host cells. RIG-I is tightly regulated, detecting viral double-stranded ribonucleic acid (dsRNA) triggering addition of scaffolding ubiquitin chains, which initiates an immune response through interferon production. Conversely, the less understood mechanism involving the Really interesting new gene (RING) finger protein 125 (RNF125), an E3 ligase, plays a pivotal role by attaching degradative ubiquitin chains onto RIG-I and shutting down RIG-I signaling through the ubiquitin-proteasome system. Though importance of RNF125 is highlighted by the fact that disruption of RNF125 function is linked to Tenorio syndrome. A rare syndrome that manifests symptoms including macrocephaly, intellectual disability, hypotonia, overgrowth, congenital heart disease, and autoimmune disease.
RNF125 comprises multiple domains, an N-terminal myristoylation site believed to aid membrane localization, RING and zinc finger 1 (ZF1) domains that assemble ubiquitin chains, two additional ZF domains and a ubiquitin interaction motif (UIM) at the C-terminus with unclear functions. The UIM likely binds ubiquitin chains to aid substrate recruitment, may bring RIG-I in close proximity for addition of degradative ubiquitin chains. In this study, a novel variant within the UIM of RNF125 was identified in an individual displaying symptoms consistent with Tenorio syndrome [(NM_017831.4):c.670G>C p.Glu224Gln]. This variant was found to not affect the E3 ligase activity or RNF125 stability, assessed using an in vitro ubiquitylation assay and in vivo expression studies, respectively. However, assessment of physical interactions using an in vitro pull-down assay, noted that the variant significantly impaired the interaction between RNF125 with scaffolding ubiquitin chains. While the precise role of the UIM in RNF125 remains uncertain, the study highlights the potential importance of binding scaffolding ubiquitin chains for RNF125's normal function. Further research is required to understand RNF125's broader roles in development and substrate recruitment. These findings enhance our understanding of the molecular basis of Tenorio syndrome and the complex interplay between RIG-I signaling, ubiquitination, and cellular homeostasis. Offering insights for the development of targeted therapies for inflammatory disorders and related conditions.